Conventionally, there are instruments that receive the light reflected from a measuring object (object to be measured) such as a target and amplified with a photo multiplier or avalanche photodiode (APD), and performs a predetermined process, to determine delay time or distance. For such instruments, it is required to receive the light in the wide range level from the strong reflected light from near distance to the minute reflected light from far away. In this case, the intensity change of the received signal easily appears in the measurement error, so an instrument that performs a delay time measurement or distant measurement by detecting a zero cross point of the damping signal that is a part of the frequency component taken out from the received light signal and by amplifying, has been proposed. (Refer to for example Patent Document 1).
[Patent Document 1] JP 7-191144 A; par. 0056-0062, 0113 to 0120, FIG. 1, and FIG. 9 (a)